Specify WebTransport initialization in more detail

[yutakahirano]

This is for #128.

• Introduce WebTransportOptions.dedicated.
• Initialize a connection with
  https://fetch.spec.whatwg.org/#concept-connection-obtain.
• (Bonus) Introduce WebTransport.Closed and reject it when the connection establishment fails.

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Preview | Diff

[yutakahirano]

@aboba @jan-ivar PTAL.

cc: @annevk

[annevk]

Looks good overall!

[aboba]

Question: How does allowPooling interact with server indications of pooling support? Is this taken into account in the fetch algorithm? For example, if the server indicates it does not support pooling, will connections to that endpoint not be pooled? With respect to the WebTransport constructor, my assumption is that there is no interaction between allowPooling and server indications (e.g. allowPooling is a preference which can be accommodated or not, based on user agent and server capabilities).

See slide 21 here for potential options for how the server indication could work.

Otherwise, LGTM.

[yutakahirano]

Question: How does allowPooling interact with server indications of pooling support? Is this taken into account in the fetch algorithm? For example, if the server indicates it does not support pooling, will connections to that endpoint not be pooled? With respect to the WebTransport constructor, my assumption is that there is no interaction between allowPooling and server indications (e.g. allowPooling is a preference which can be accommodated or not, based on user agent and server capabilities).

That depends on the resolution of ietf-wg-webtrans/draft-ietf-webtrans-http3#34. When I was writing this I was imagining
negotiation with SETTINGS frame. I'm leaning to landing this now and update the logic after the resolution.

[aboba]

That depends on the resolution of ietf-wg-webtrans/draft-ietf-webtrans-http3#34. When I was writing this I was imagining
negotiation with SETTINGS frame. I'm leaning to landing this now and update the logic after the resolution.

@yutakahirano Sounds good.